Researchers analyzed data from more than 1,500 people, average age 70, who were depression-free and had their thyroid activity assessed at the start of the study.

Over eight years of follow-up, people with thyroid glands that were more active than average -- but still within the normal range -- were more likely to develop depression than those with lower levels of thyroid activity within the normal range, the investigators found.

"These results provide insight into the powerful effects thyroid activity can have on emotions and mental health," Dr. Marco Medici, of the Erasmus Medical Center in the Netherlands, said in a news release from the Endocrine Society.

Hormones produced in the thyroid gland control the rate of many bodily activities.

Previous research found associations between markedly high or low thyroid activity and increased risk of depression. However, this is the first study to find a link between depression and higher levels of thyroid activity within the normal range, the study authors pointed out in the news release.

The study only found an association between above-average thyroid activity and depression risk. It did not prove cause-and-effect.

The results, published online Feb. 20 in the Journal of Clinical Endocrinology & Metabolism, suggest that even minor changes in thyroid function may have mental health effects similar to those associated with major changes, Medici noted.

"This information could influence the process of diagnosing and treating depression, as well as treatments for individuals with thyroid conditions," he said.